Multilayer plastic chip capacitor

ABSTRACT

Multiplayer chip capacitors and method of making whereby twoside metalized plastic strip material is wound in annular hoop form and opposite sides of hoop are pressed together to bond layers together. The metalized plastic strip is pre-coated with a very thin coating of a heat sealable material such as plastic which functions as a capacitor dielectric and also as a bonding agent. The marginal edges of the pressed hoop are sprayed with molten aluminum at high velocity which makes contact with the edges of the thin metalized layers and penetrates the plastic coating to also provide contact with marginal portions of the metalized electrode coatings. The flattened hoop is then sliced into a plurality of capacitors which can be made to have any value desired over a large range by cutting them to a particular width. Values can be controlled very precisely by first cutting and testing one capacitor and then altering the width of later cut capacitors relative to the tested one. The uncut capacitors can be stored and cut to value as needed.

Unlted States Patent 1 1 ,654,532 Rayburn 1451 Apr. 4, 1972 [54]MULTILAYER PLASTIC CHIP Primary Examiner-E. A. Goldberg CAPACITORAttorney-Robert W. Beart, Michael Kovac, Barry L. Clark dJ k R. H l [72]Inventor: Charles C. Rayburn, Falls Curch, Va. an ac a verse [73]Assignee: Illinois Tool Works Inc., Chicago, Ill. ABSTRACT 22 Filed; May25, 1970 Multiplayer chip capacitors and method of making wherebytwo-side metalized plastic strip material is wound in annular PP N0540,012 hoop form and opposite sides of hoop are pressed together to bondlayers together. The metalized plastic strip is pre-coated s2 u.s.c1..3l7/258,3l7/260 317/261 with a mating 0f a heat Scalable material Suchas [51] Int. Cl. .Jifllg 1/14 Plastic which functions as a capacitordielectric and also as a 581 Field of Search ..317/242, 260,258, 261bnding 98 The marginal the F l P are sprayed with molten aluminum athigh velocity which makes [56] Reerences Cited contact with the edges ofthe thin metalized layers and penetrates the plastic coating to alsoprovide contact with UNITED STATES PATENTS marginal portions of themetalized electrode coatings. The flattened hoop is then sliced into aplurality of capacitors 1,173,452 Mfill'OWSky which can be made to havey value desired over a large 2'921246 1/1960 Peclf "317/260 range bycutting them to a particular width. Values can be 3'2l4657 10/1965controlled very precisely by first cutting and testing one valley""317/258 capacitor and then altering the width of later cut capacitorsViellerman ..3 1 7/260 relative to the tested one. The uncut capacitorscan be stored and cut to value as needed.

2 Claims, 19 Drawing Figures Patented April 4, 1972 3,654,532

2 Sheets-Sheet 1 IN VEN TOR Char/es 6. R0 burn BY %Z&

His Afr'ys Patented April 4, 1972 3,654,532

2 Sheets-Sheet 2 INVENTOR. Char/es C. Rayburn Mari His Aff'ys 1.MULTILAYER PLASTIC CI-IIP CAPACITOR BACKGROUND OF THE INVENTION A rolledcapacitor made from two-side metalized polyester film material, having aheat sealable coating of plastic material applied to each of themetalized layers for holding the capacitor together and to exclude airfrom between the layers is described in U.S. Pat. No. 3,214,657..Although the rolled capacitor described in the aforementioned patent hasmany advantages, it has certain disadvantages common to all rolledcapacitors in that each capacitor must be separately rolled and mountedfor lead attachment. Furthermore, where different values of capacitanceare desired, the length and/or width of the region of overlappedelectrodes must be varied during the winding of the capacitors. Anotherdisadvantage of a rolledtypecapacitor is that it cannot be directlyconnected to a printed circuit board or substrate without first havingleads attached to it.

In U.S. Pat. No. 2,731,706, a method of making capacitors is shownwherein a pair of strips of one-side metalized plastic film are wound inannular form about a forming ring. After several convolutions of filmare placed on the forming ring, the ends of the film are sealed and theforming ring, with the convolutions of film attached to it is removedfrom the winding machine and subjected to several operations includingthe cutting apart of individual capacitors.

It is an object of this invention to provide a plastic chip capacitorwhich can provide sufficient capacitance in a given volume so as tocompare favorably with ceramic chip capacitors or wet Tantalumelectrolytic capacitors.

It is a further object of the invention to provide a plastic chipcapacitor which, as compared to high K multilayer ceramic capacitors,offers a lower voltage coefficient, a lower temperature coefficient,freedom from capacitance aging, and self-healing properties.

An additional object of the invention is to provide a chip capacitorwhich will have a very low material cost, will provide excellent yieldcontrol, shorter production cycles, lower equipment investment and lowermaterial inventory cost than other types of capacitors now available.

The preceding and other objects are obtained by the capacitor of thepresent invention and the method by which it is made. The capacitorcomprises a plurality of layers of twoside metalized plastic dielectricmaterial which has had a very thin coating of dielectric such as plasticapplied to at least one of its metalized surfaces. For ease ofmanufacture, it is preferable that the plastic coating material extendsto the edges of the capacitor. Since the metal electrodes which areformed by vapor depositing aluminum are extremely thin, it is obviousthat there is extremely little electrode surface exposed at the end ofthe capacitor to which a terminal may be attached. lt is also obviousthat good contact must be made between each of the alternate electrodelayers which extend to a common end of the capacitor in order to placeall of the electrodes in the electric field. I have found thatsatisfactory contact to each layer can be made by subjecting the ends ofthe capacitor to a high velocity spray of molten metal, preferablyaluminum, which embeds itself in the plastic coatings between themetalized layers so as to contact the surface as well as the ends of theelectrodes, but does not substantially penetrate the plastic dielectricstrip so as to permit a shorting out of the electrodes of oppositepolarity thereon which are spaced interiorly of the ends of thecapacitor.

The preferred method for making the plastic chip capacitor as describedherein is to wind the plastic coated metalized dielectric material abouta large diameter mandrel so as to form a hoop. The hoop may be heatsealed along a line to anchor the outermost convolution. After windingthe hoop, the hoop is placed between a pair of press platens whichfiatten it under pressure. The platens are then heated to bring theheat-fusible thin plastic film coating between the electrode layers to afusing temperature. During the heating of the capacitor layers, a lightpressure is maintained which is sufficient to exclude air and bring thesurfaces into an intimate contact and yet is low enough to prevent thethin plastic coating from extruding and thus uncontrollably varying itsthickness and thus, its capacitance. After fusion occurs, the pressureis maintained and the platens are cooled, following which the capacitorflat is removed as a solid block.

The capacitor fiat is then subjected to a metal spraying operationwherein an oxygen-acetylene flame melts continuously fed aluminum wirewhile a concentric, high velocity, air stream breaks up the moltenaluminum into particles about 0.001 inch in diameter and impels themagainst the thermoplastic capacitor flat. The capacitor flat is thencleared by subjecting it to a DC voltage equivalent to about one quarterof its intended rated DC voltage to burn open any major shorting areas.This procedure is then followed by a high DC voltage, usually twice therated voltage, to complete the clearing process. After'clearing thecapacitor fiat, its capacitance value is measured since the two ends ofthe fiat, which are scrap, can be considered as forming the two halvesof a cylindrical capacitor, the relative area of the cylindricalcapacitor as compared to the flat remaining area can be readily figuredmathematically and its value subtracted from the total value read inorder to permit the capacitance per unit of length of the flat area tobe determined.

The capacitor flat may then be cut into a plurality of identicalindividual chips which will have a very uniform value since the activearea, film thickness and dielectric constant are very uniform within agiven capacitor flat. To achieve a predetermined value of capacitance,the first chip or two out from a fiat may be value sampled and a vemierstop on a cutting mechanism adjusted so that the value of subsequentlycut chips will achieve the predetermined target value. After the chipsare cut, their values may be adjusted slightly downwardly if desired bythe application of a high voltage in order to burn back the electrodesurface and thus reduce the electrode area. Cutting can be done by manytypes of equipment, such as a high speed saw, a hot wire, a high speedfriction blade or a shear. Since each of these cutting techniques wouldpresent the possibility of wiping electrode material between electrodesalong the cut edges, it is necessary to clear the parts again aftercutting to burn open the conductive paths. The additional step ofclearing after cutting can be avoided if the metalized material wound informing the capacitor is appropriately demetalized in those areas wherecutting will take place.

By winding the hoop from which the chips are cut from metalized stripshaving multiple metalized patterns which are staggered transversely, itis possible to achieve a capacitor fiat which may be cut in twodirections to form a large plurality of chips.

Where the chips are formed by cutting the fiat in two directions, it is,of course, necessary to apply the terminations after the cutting takesplace. This may be done by stacking the cut chips so that one edge ofeach chip lies in a plane which is then subjected to a metal sprayingoperation. By spraying aluminum on the edges of the capacitor containingthe electrode terminals and continuing the spray around an adjacent endand then applying a coating of solder onto the end, it becomes possibleto solder the chip to a tinned substrate pattern by the momentaryaddition of heat and solder in the localized area on the said end edgeof the capacitor. The solder coating permits the connection between themetalized electrodes and the sprayed aluminum to remain cool duringsoldering and thereby minimizes any chance of thermal damage.

In order to enhance the degree of contact of the sprayed aluminumcoatings to the extremely thin electrodes, the capacitor chips may becut on an angle rather than perpendicular to the planes of the electrodelayers. Such angled cutting increases the width of the exposed edgesurface and also presents the electrodes at the edge in step-wisefashion so that the sprayed metal will contact not only the outer edges,but a portion of the marginal length of each electrode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross-section of atwo-side metalized and plastic coated dielectric strip used in makingthe capacitor of the invention;

FIG. 2 is a view similar to FIG. 1 showing a modified form of strip witha marginally recessed plastic coating;

FIG. 3 shows a fragmentary cross-section of a marginal portion of amultilayer stack capacitor made from the strip shown in FIG. 1 after aspray metal coating has been applied thereto;

FIG. 4 is a view similar to FIG. 3 showing the application of a spraymetal coating to a stack capacitor made from the strip shown in FIG. 2;

FIG. 5 is a perspective view showing the strip material of FIG. I woundinto a hoop;

FIG. 6 is a view illustrating the flattening under pressure of the hoopof FIG. 5;

FIG. 7 is a perspective view showing metal spraying of the marginaledges of a plurality of the flattened hoops shown in FIG. 6;

FIG. 8 is a perspective view illustrating the measuring of thecapacitance of one of the flattened hoops which are metal sprayed inFIG. 7; 1

FIG. 9 shows a perspective view of a device for cutting the flattenedcapacitor hoop into a plurality of individual chip capacitors;

FIG. 10 is a perspective cross-section taken on the line 10- 10 of FIG.11 showing a multilayer capacitor flat made from a wide web of two-sidemetalized and plastic coated dielectric material which differs from thestrip of FIG. 1 in that it has multiple metalized patterns adapted to becut longitudinally along the dotted lines;

FIG. 11 is a perspective view of a flattened capacitor hoop wound frommaterial shown in cross-section in FIG. 10 with the cutting linesdotted;

FIG. 12 is a plan view of a modified form of multilayer capacitor flatwith the top layer of plastic removed to expose the electrode plates,which is similar to the capacitor flat of FIG. 10 except that thelongitudinal metalized patterns are interrupted by cleared areas;

FIG. 13 is a cross-sectional view taken on the line 13-13 of FIG. 14transversely of the length of the dielectric strip;

FIG. 14 is a cross-sectional view taken along the line 14-14 in FIG. 12;

FIG. 15 is a perspective view of a capacitor chip formed by cutting theflattened hoop shown in FIGS. 12-14 in two directions and applying asprayed metal coating to the edges having exposed electrodes;

FIG. 16 is a view similar to 15 showing the application of a sprayedmetal coating not only to the exposed electrodes, but partially aroundone end of the capacitor and the application of a coating of solder tothat portion of the sprayed coating which is not in direct contact withthe electrodes;

FIG. 17 is a longitudinal section through a modified form of multilayercapacitor flat having a series of cleared longitudinal lanes andtransverse cutting lanes which are staggered to present alternateelectrodes;

FIG. 18 is a diagrammatic representation of the application of a sprayedmetal coating to the offset electrodes of capacitor chips cut from thecapacitor flat of FIG. 17; and

FIG. 19 is a perspective view of an individual capacitor chip cut fromthe capacitor flat shown in FIG. 17 and sprayed with metal on itsbeveled ends.

DESCRIPTION OF THE PREFERRED EMBODIMENT coatings I4 and 16 are-thencoated with an extremely thin layer of dielectric 18, 20 such aspolystyrene or polycarbonate plastics which extends to their marginaledges 22, 24. When a plurality of the composite layers 10 are pressedtogether and heated in a manner to be hereinafter described, the thinplastic coatings 18, 20 fuse together to form a monolithic capacitorhaving a much higher capacitance than an ordinary polyester capacitorsince the plastic coatings 18, 20 have a combined dielectric thicknesssubstantially less than the thickness of the substrate 12. Typicalthicknesses of the various materials are 0.0002 inch for the polyestersubstrate, 0.000001 for the metallic electrode layers 14, 16 and0.000025 inch for each of the thin plastic coatings 18, 20.

The composite film strip 10 shown in FIG. 2 is identical to that shownin FIG. 1 except that the metalized dielectric strip 12' is maskedduring application of the plastic coatings 18', 20 so as to extend shortof the marginal edges 22, 24.

FIGS. 3 and 4 illustrate the cooperation of a sprayed metal coating 40or 40 with the marginal edges 22 or 22 of a multilayer stack ofcomposite layers 10 (FIG. 3) or 10 (FIG. 4). In FIG. 3, it can be seenthat the lower plastic coating on one composite layer 10 has been fusedtogether with the upper plastic layer of the adjacent composite layer 10to form an integral fused coating 19 due to application of heat andpressure to the stack. The sprayed metallic coating 40 can be seen asslightly penetrating the edges of the substrate 12 and much more deeplypenetrating the softer plastic 19 so as to become embedded in it andpenetrate it so as to make contact with portions of the flat surface ofelectrodes 14 as well as their ends. In FIG. 4 it can be seen that thepenetration of the metal spray 40 is somewhat deeper than that in FIG. 3since the plastic coatings 18, 20, and thus the fused coating 19', arerecessed. This embodiment would of course provide substantially moredirect contact between the metal spray particles 40 and the surfaces ofelectrodes 14, but at the expense of providing masking during thecoating of the electrodes.

To form the capacitor of the present invention, a strip of the compositefilm 10 is wound in a plurality of convolutions around either a circularcollapsible mandrel (not shown) so as to form an annular hoop 30 shownin FIG. 5, or about other structure such as a pair of spaced apart pinswhich would provide a rather flattened hoop (not shown). In order tohold the hoop together after winding is completed, the outer convolutionof the hoop 30 may be heat sealed along a line 32 to the adjacentconvolutions. Suitable structure for performing such sealing is shown inU.S. Pat. No. 2,950,070. Although heat sealing, in accordance with theaforementioned U.S. patent, wherein cutting of the web takes placesimultaneously, eliminates the necessity for a separate cuttingoperation, it is obvious that other methods of sealing and cutting couldbe used, including the use of adhesives or tape.

FIG. 6 illustrates the processing step of compressing the hoop 30between a pair of press platens 34, 36 so as to form a capacitor flat30. In order to eliminate the seal line 32 from having any effect on thecapacitor chips 30b which are to be cut from the flat 30, the hoop 30'is pressed so that the seal line 32 will end up in a scrap area of theflat 30 at one of its ends. To form the flat 30, the platens 34, 36 areheated so that the plastic layers 18, 20 will be slightly softened so asto flow together and bond to each other without affecting the substratedielectric material 12. The amount of heat and pressure applied varies,depending on the particular materials used. However, it is importantthat the pressure and heat be sufficient to exclude air from between thecoated layers 18, 20 but not be so great as to cause such layers 18, 20to extrude outwardly since such a condition could cause the metalliclayers l6, 14 which they separate to short out or could vary thethickness of the final fused layer 19 so as to alter the value ofindividual capacitor chips cut from the flat 30. After fusing, theplates 34, 36 are cooled before the pressure is released.

FIG. 7 illustrates the step of applying a metal sprayed coating 40 witha gun 42 to the edges of a plurality of capacitor flats 30 held byclamps 44, 46. The coating 40 becomes embedded in the plastic layers 19as shown in FIGS. 3 and 4 so as to electrically connect each of theelectrode layers at one marginal edge of the flat. In order for theparticles of metal spray 40 to become embedded in the plastic layer 19,and contact not only the terminal edge portions of the electrodes 14,but also a portion of their flat surfaces, the metal spraying must bedone at rather high velocity. Suitable commercially available materialsand equipment and settings which have proved suitable, are as follows: aMetco 4E Gun, a Metco Flow Meter, and Metco gauge aluminum wire. Thewire is fed at a rate of 13 plus or minus 2 feet per minute with the gunnozzle being approximately 5% inches from the capacitor. During thespraying, the flow meter readings should be approximately 50 for theair, 15 for the oxygen, and 15 for the acetylene. An oxygen-acetyleneflame melts the aluminum wire and a concentric, high velocity, airstream breaks up the molten aluminum into particles about 0.001 inch indiameter and impels the hot particles against the thermoplasticcapacitor flat.

In FIG. 8 the processing step of clearing is illustrated. The capacitorflat 30 is cleared after metal spraying by subjecting it to an ACvoltage passing through the contacts 52, 54 which is equivalent to aboutone-fourth of its intended rated DC voltage. This low impedance sourceburns open any major shorting areas. This procedure is followed by ahigh voltage DC, usually twice the rated voltage, to complete theclearing process.

After clearing the capacitor flat 30, its capacitance value is measuredby apparatus (not shown). Since the two semi-circular ends a F IG. 9) ofthe capacitor flats 30 combine to form the two halves of a cylindricalcapacitor which is scrapped, it is a simple mathematical exercise tocompare the cross-section of the scrap capacitor cylinder to the flatportion remaining after the ends are removed to determine thecapacitance of the elongated remaining flat portion. An additionalcalculation permits the capacitance per unit of length to be determined,which value can be used to determine the width to which the capacitorsshould be cut in cutting apparatus 56 (FIG. 9). The cutting apparatus 56includes a cutter blade 58 which cuts individual capacitor chips 30bfrom a capacitor flat 30 as the capacitor flat is pushed into the bladeby pusher member 60. A vernier stop member 62 permits the width of eachcapacitor chip 30b to be precisely controlled so that successivecapacitors will have substantially identical values. If desired, thefirst few chips cut from a particular capacitor flat 30 may be valuesampled, and the vemier stop 62 readjusted to move the value ofsubsequent chips to center on a predetermined target value. The chipsmay also be adjusted downwardly in value by the application of a highvoltage to burn back the electrode surface, thereby reducing theelectrode area by the desired amount. Although a saw is shown forcutting the capacitor flat, it is obvious that other techniques such asa hot wire, a high speed friction blade, a shear or other device capableof cutting a strip of plastic with a crusty aluminum surface, would besuitable. Since the cutting techniques just mentioned cause electrodematerial to be wiped together along the the cut edges, it is necessaryto clear the parts again to burn open the conductive paths.

FIGS. 10 and 11 illustrate a more efficient production system wherein awide web of composite film material 110 having metalized patterns 114,116 coated with plastic 118, 120 which are continuous in a longitudinaldirection and staggered transversely is stacked so as to form amultilayer flat bonded together by fused layers 119. Longitudinal cutsmade on the lines 131 will produce a plurality of capacitor flats 130which are individually identical to the capacitor flats 30 previouslydiscussed.

FIGS. 12-14 illustrate a system of electrode patterns differing fromFIG. 10in that the electrodes are interrupted longitudinally so as toprovide clear cutting lanes 132 at regular intervals. This system ofpatterns eliminates the requirement of clearing after severing. To formchip capacitors from the wide web of flat portions 230, the compositestrips 210 can be cut at regular. intervals, stacked and then heatpressed and cut. Since the films are transparent except in theirelectrode areas,

'around the chamfered the system lends itself to photoelectricregistration control. With the electrode pattern system shown in FIGS.12-14, the fabricated strips may be cut first longitudinally into flats230, sprayed as in FIG. 7, and then cut transversely into chips as shownin FIG. 15. Or, the fabricated strips may be cut in either direction,than the other to provide an unsprayed chip 23012. This unit may then bestacked with hundreds more to spray the edges after cutting so as tocontact the electrodes 214, 216. This latter procedure prevents thecutting operation from breaking any electrical bonds between the sprayedmetal 240 and the evaporated electrodes 214 or 216.

FIG. 16 shows a further refinement in the sprayed area which connectsthe sprayed particles 240 to the evaporated electrodes 214 of a chip230b. The sprayed area 240 is carried comer 230a and almost across theend 230d. The electrode edges are sprayed with aluminum, then theportion of aluminum on end 230d is over-sprayed with solder 241. Thisprocedure allows the end of the capacitor to be soldered to a tinnedsubstrate pattern by the momentary addition of heat and solder in thislocalized area. The connection between the metalized electrodes 214 andthe sprayed aluminum 240 remains cool, thereby minimizing any chance ofthermal damage to this somewhat tenuous connection as the capacitor isconnected into its substrate position or as lead wires are assembled.

FIGS. 17-19 shows a modified arrangement for producing a chip whichpresents more area to the aluminum spray, thereby giving a contact oflower resistance and higher reliability. The individual composite filmwebs 310 are metalized with clear longitudinal lanes which are stackedin exact register. The transverse clear lanes 332 are staggered topresent alternate electrodes at consecutive cut lines. After thecapacitor flat 330 is cut on an angle in the cleared area 332, theindividual cut chips are stacked as shown in FIG. 19 and metal pliedthereto from a metalizing gun 342. Since, wound metalized capacitor, achip must have every electrode contacted at each layer, the beveled cutpresents greater electrode area to the spray to increase the contactarea and improve the contact.

FIG. 20 shows a completed beveled chip 330b which has a sprayed metalcoating 340 on its exposed electrodes and a solder over-spray coating34] which functions the same as the coating 241 in FIG. 16.

I claim:

1. A multilayer stack capacitor comprising a plurality of flat layers ofplastic strip, a vapor deposited metalized coating on each side of saidlayers of plastic strip, said metal coatings on each side of said layersoverlapping each other and extending to the side marginal edges of saidlayers except for an unmetalized portion along opposed margins, saidlayers of metalized plastic strip being affixed firmly to each other bya coating of plastic dielectric, which is substantially thinner thansaid plastic strips, applied to each of the metalized surfaces of eachof said plastic'strips, said plastic dielectric being heat softenable sothat heat and pressure applied to said stack to bond its layers togetherwill soften said thin plastic dielectric coatings to cause them to fusetogether as a dielectric layer which is substantially thinner than thelayer of plastic strips and to bond said layers together withoutdamaging said plastic strip layers, and a plurality of particles ofsprayed molten metal embedded in each of the opposed margins of saidcapacitor to a depth less than the width of said unmetalized marginportions so as to penetrate said thin plastic coatings and contact theflat marginal metalized surfaces on each of said layers withoutsubstantially penetrating said plastic strip which provides efficientelectrical connection between a terminal and the metalized coating. 7

2. The capacitor of claim 1 wherein the marginal edges of said striplayers of capacitor containing said sprayed metal particles are inplanes formed at an acute angle to the planes of said strip layers so asto expose a larger area of said marginal'metalized surfaces to saidsprayed particles than is possible when said marginal edges are inplanes normal to the planes of said layers.

2. The capacitor of claim 1 wherein the marginal edges of said striplayers of capacitor containing said sprayed metal particles are inplanes formed at an acute angle to the planes of said strip layers so asto expose a larger area of said marginal metalized surfaces to saidsprayed particles than is possible when said marginal edges are inplanes normal to the planes of said layers.